In mounts for optical systems, it is often desirable to observe basic principles of kinematics. A body in space, such as a lens or mirror, has six degrees of freedom or ways in which it may move: translation along the three rectangular coordinate axes, and rotation about these three axes. A body is fully constrained when each of these possible movements is singly prevented from occurring. However, it is sometimes desirable in an optical system for some degrees of freedom to be allowed, and so semikinematic methods can be used.
Meanwhile, locating an optical element and maintaining its position relative to other optical elements in an optical system is difficult. Moving an optical element from one position to another and reestablishing alignment is extremely difficult especially over adverse environmental conditions. More particularly, for benign environments like a laboratory or medical facility, it is more straightforward to position an optical element like a secondary mirror accurately. Since there is no external environmental input like vibration shaking the structure, the optical alignment is maintained. In contrast, for optical sensors subjected to adverse environmental conditions like shock and vibration, conventional techniques for holding and positioning an optical element can limit the optical performance of the sensor. When subjected to vibration, like the turbulence from an aircraft, if the structure holding the optical elements is not rigid, it will oscillate at a characteristic amplitude and frequency, and the optical image will blur due to motion of the optics. A key performance specification for optical sensors subjected to environmental disturbances is line of sight (LOS) stability. Line of sight stability is proportional to optical blur. For an optical element that must be moved into different positions, the structure and positioning features must be very stiff and/or deterministic to prevent energy from the environment from disturbing the position of the optical elements.
FIGS. 1A and 1B illustrate an example of a prior art technique for holding and positioning an optical element in a stabilized platform.
As shown in FIG. 1A, an optical element 102 is mounted to a structural arm 104 that pivots on hinges. The structural arm 104 is actuated by a motor (not shown). To minimize optical obscuration, the size of the arm is minimized. As shown in more detail in FIG. 1B, arm 104 is rotated about hinges 106 consisting of ball bearings or bushings and a shaft. Arm 104 might be positioned using an encoder or potentiometer. Hard stops can be used to determine the final position of the arm and hold it in place.
Prior art techniques such as that illustrated in FIGS. 1A and 1B suffer from many problems. Hinges made from bearings or bushings have limited overall stiffness and will limit the dynamic response of the optical system. The relatively narrow shafts that ride in the bearings also have limited stiffness. While the structure that mounts the optics can be very stiff, the hinge/bearing structure limits the overall stiffness of the assembly. Stops used to position the structural element at the end of travel typically can not restrain all six degrees of freedom. The final position of the optical element is indeterminate at best and has limited stiffness.
High performance optical assemblies for use in stabilized platforms that do not limit the imaging performance of the sensor thus remain a highly desirable need in the art.